The present invention relates to a shielded cable and a tape for constructing the same.
The design requirements for information transmission cables, such as CATV cables include: low signal loss, shielding from extraneous electrical fields, cable flexibility and susceptibility to economical manufacture. Often, when it is desired to transmit high frequency signals, especially television signals, with a low loss, shielded cables are employed. A shielded cable usually includes a core of one or more insulated conductors enclosed within at least one conducting layer. The shielding resists signal leakage from the core and eliminates or reduces the interfering effects of extraneous electrical fields.
In order to provide flexibility, prior shielded cables have utilized a braided outer conductor or shield of copper or similar conductive material. However, braided shields, because of the spaces between the wire braids have the disadvantage of providing less than 100% coverage of the cable core. The effectiveness of a braided shield may be increased by more fine braiding or by a double layer of braid. Unfortunately, such expedients frequently result in the cable being excessively stiff, large or heavy; such a shield may be difficult to cut and terminate in a suitable electrical connection. Moreover, the greater amount of material required in such a construction increases the cost of the cable.
It is more economical to construct a cable by wrapping flexible tape with a conducting layer about an insulated conductor core. The tape may be wound helically about the core or wrapped longitudinally. It is highly desirable to provide double or multiple shielding to better retain signals transmitted by the core and insulate the core from extraneous signals and to provide backup shielding should one of the layers rupture by flexing of the cable or otherwise. Since it lacks the spaces inherent in a braiding, a tape provides very effective shielding, stopping signals which would frequently leak through a braided shield.
It is desirable that the conducting layer or layers of the tape be quite thin to minimize expense, stiffness and bulk. However, very thin conducting layers do not have the strength and durability required in most applications for flexible cables. Therefore, it is convenient to provide a backing for the thin conducting layer to enhance its integrity. Thus, tapes are currently manufactured with a conducting layer over a backing or with a strength-giving layer sandwiched between two conducting layers. Such tapes are relatively inexpensive and convenient to handle.
The dielectric character of most suitable backing materials causes a problem in completing electrical shields using these tapes. If layered shielding tape is wound around a cable so that the ends overlap, the backing prevents a conducting layer from contacting itself in the region of overlap. This failure of contact leaves an electrical gap or slot in the shielding which permits signal leakage to and from the core. The resulting power loss and signal interference is referred to as the "slot effect".
One method of dealing with the slot effect is to fold back an edge of the tape in the region of overlap so as to permit a conducting layer to touch itself and thereby close the slot. However, there are several disadvantages to this approach. When it is used with tape having a single conducting layer, the approach does not provide multiple shielding. Thus, if the cable cracks due to flexing or other causes, the effectiveness of the shielding is greatly reduced. In the case of a tape with two conducting layers sandwiching an insulating layer, this method may fail to provide a ground connection between the conducting layers. The lack of such a ground connection makes it necessary to create more complex connections during cable termination or hook-up; otherwise, the advantages of the multi-shielding are lost. Furthermore, the narrow folded edge is difficult to handle reliably during manufacture and termination of the cable. Additionally, the narrow folded edge creates a longitudinal lump along the cable which is much stiffer than the neighboring areas; the stiffness differential on both sides of the lump increases the vulnerability of the shielding to rupture during flexing.
A more effective method of dealing with the slot effect is to provide one or more drain wires upon the periphery of the tape as disclosed in U.S. Pat. No. 3,927,247. However, such cable requires additional materials and manufacturing steps during cable manufacture, thus adding to the bulk of the cable and to manufacturing costs. In the case of the tape with two conducting layers, the disclosed drain wires would leave a slot in the inner of two conducting shields. Finally, the disclosed drain wire approach is not directly applicable to more than two layers of shielding.